1. Field of the Invention
The present invention relates to metabolic gas monitoring apparatus and methods, and in particular, to a novel apparatus and method for measuring respiratory gases in newborn infants.
2. The Prior Art
Small newborn infants, particularly those born prematurely, often have insufficient lung development with the result that they suffer from respiratory distress. It has been found that vigorous mechanical ventilation of infants with various types of lung disorders significantly improves their expected rate of survival. However, one of the difficulties with mechanical ventilation of newborn infants is that it is extremely difficult to accurately monitor the ventilation. As a result of this problem it has been difficult to even decide how to properly set a mechanical ventilator so that adequate ventilation occurs without over or under ventilating the infant. Indeed, one of the dangers of mechanical ventilation has been the potential for hyperinflation of the infant's lungs, which may itself result in lung damage. Thus, to date rather cumbersome and inaccurate methods such as chest roentgenograms, arterial blood gas determinations and/or physical examination have been used in order to estimate whether the ventilator was set properly.
Apart from the decision of how to set the level of ventilation, analysis of the respiratory gas of a newborn infant is important for other reasons. By measuring the level of oxygen and carbon dioxide in the expired gas and comparing these levels with the levels of oxygen and carbon dioxide in the inspired gas, it is possible to determine metabolic rate using calculations which constitute a method known as indirect calorimetry. This information is highly useful in diagnosing various problems and also in developing an appropriate diet for the newborn infant. This information may be particularly important in the case of very premature or critically ill infants because improper nutrition at the early stages of life may cause permanent disorders and impede growth.
Accurate measurement of the respiratory gases of a newborn infant can also be used to estimate blood flow, which otherwise could only be roughly estimated using charts and tables, which is a much less accurate method. By carefully monitoring respiratory gases it is also possible to gain a good understanding of changes in cardiovascular function and tissue perfusion, which are also important considerations in clinically diagnosing and caring for newborn infants who may be critically ill.
In summary, it has long been recognized that accurate monitoring of respiratory gases would provide much information that would be highly useful to doctors, nurses and technicians caring for critically ill infants. However, to date the state of the art does not provide any apparatus or methods which are very satisfactory. Some attempts have been made to monitor respiratory gases in the case of infants which are well enough developed to breathe without the use of mechanical ventilation. In such cases the infant's head is typically placed under a hood into which an oxygen enriched gas is introduced. Attempts to monitor the respiratory gases of infants in this type of situation have typically used devices which evacuate the air from the hood. The difficulty with this procedure is that a newborn infant typically expires a relatively small volume of gas into the chamber of the hood, which is then mixed with the remaining gas in the hood before it is evacuated. Thus, this procedure is not very accurate.
Moreover, in the case of critically ill newborn infants mechanical ventilation must be used since typically these infants are incapable of breathing on their own. Especially in the case of a mechanically ventilated infant, the volume of gas expired by the infant is so small that it is extremely difficult to accurately monitor and detect the expired gases once they enter back into the ventilation stream. Thus, even though some attempts have been made in the prior art to place a monitoring apparatus into the ventilation stream downstream from the patient, such attempts have been largely unsuccessful in providing accurate measurement of the respiratory gases. Thus, to date there has not been devised a satisfactory apparatus or method for accurately monitoring the respiratory gases of newborn infants, especially in the case of those needing mechanical ventilation.